C++: Respond to review comments.

Author: MathiasVP

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -1642,6 +1642,10 @@ predicate localInstructionFlow(Instruction e1, Instruction e2) {
 
 /**
  * INTERNAL: Do not use.
+ *
+ * Ideally this module would be private, but the `asExprInternal` predicate is
+ * needed in `DefaultTaintTrackingImpl`. Once `DefaultTaintTrackingImpl` is gone
+ * we can make this module private again.
  */
 cached
 module ExprFlowCached {